The present invention relates generally to insulated gate field effect transistors (IGFETs) such as metal oxide semiconductor FETs (MOSFETs). More particularly, it relates to vertical, double diffused MOSFETs (VDMOS) and vertical, grooved MOSFETs (VMOS) used in power applications.
A conventional IGFET is a unipolar transistor in which current flows from a source region, through a channel in a body region, to a drain region. The source, channel and drain regions are of N or P type conductivity and the body region is of opposite conductivity type. The channel is induced (in an enhancement type device) or removed (in a depletion type device) by means of an electrostatic field produced by charges on a nearby gate electrode. The gate electrode typically lies between source and drain electrodes (disposed respectively on the source and drain regions) and in a MOSFET device it is insulated from the semiconductor surface by an oxide layer.
In vertical MOSFETs, the source and drain electrodes are on opposite semiconductor surfaces and create current flow which is substantially vertical (perpendicular to the semiconductor surfaces) through the device. In vertical VDMOS devices, the substrate is substantially planar and the gate is on the same semiconductor surface as the source electrode. In vertical VMOS devices, a groove extends into the substrate from one surface and the gate is disposed on the surface of the groove.
Both varieties of conventional vertical MOSFETs are thus three layer devices having source, body and drain regions. In the present invention, a minority carrier injecting fourth semiconductor region, hereinafter referred to as an anode region, is introduced to a vertical MOSFET (or IGFET) structure. Although this creates a four layer device, the conductivities and geometries of the four semiconductor regions are manipulated so as not to form a regenerative thyristor. Bipolar effects are deliberately minimized. The four layer device of the present invention instead performs substantially as a field controlled transistor having improved operating characteristics compared to conventional vertical MOSFETs.